This invention relates to photo-electric conversion devices in which the light receiving elements in a light receiving element array utilizing an accumulation effect are scanned to output charges corresponding to the quantities of incident light accumulated in the elements, and more particularly to a photo-electric conversion device in which the accumulation time is varied according to information regarding the incident light.
In the case where a light receiving element array using an accumulation effect is utilized for an automatic focusing device of a camera or the like, the range of illumination intensity used thereby is considerably wide. Therefore, if the accumulation time of the light receiving element array is made constant, it is impossible to obtain a sufficiently wide range of illumination intensity. Furthermore, since the output level varies with the illumination intensity, the processing circuit is unavoidably intricate.
In order to eliminate such difficulties, a system has been proposed in which the accumulation time of the light receiving element array is varied according to information regarding the incident light. For instance, a system is disclosed by in Japanese Patent Application Laid-Open No. 140510/1976 which comprises a photo-electric conversion block which includes a light receiving element array for accumulating information on incident light through an accumulation effect, a circuit for scanning the light receiving elements in the light receiving element array successively, and a detector for generating a signal corresponding to the average value of the light intensities applied to the light receiving elements; a circuit for producing an output having a frequency corresponding to the output of the detector in the block; and a circuit for converting the output of the oscillating circuit into a scanning signal, so that when the average value of the light intensities applied to the conversion block is large, the scanning operation is carried out at a relatively high speed to prevent the operating regions of the elements from being saturated; and for outputting information from the elements successively.
According to this system, even if the average level of the light receiving element array is changed, it is possible to output the information from the elements effectively within the dynamic range. However, in this system, when high intensity light is applied to several bits in the light receiving element array and low intensity light is applied to the remaining bits, the average level is determined by the low intensity light. Accordingly, the accumulation time is increased, and the several bits to which the high intensity light is applied are saturated. Moreover, the known system has a further drawback, in that at a low intensity of illumination, sufficiently high linearity cannot be obtained because of a leakage current in the photo-electric conversion block.